Field of the Invention
The present invention relates to a fixing mechanism of a laser holder which is to be mounted on an optical scanning device.
Description of the Related Art
In the related art, an optical scanning device used in an image forming apparatus of an electrophotographic system is configured to deflect a laser beam emitted from a laser beam source such as a semiconductor laser by a deflection unit such as a rotary polygon mirror or the like and to guide the deflected laser beam to a photosensitive member with a lens or a mirror. The image forming apparatus develops an electrostatic latent image formed on the photosensitive member by being exposed to the laser beam with toner, transfers the developed toner images to a recording medium, thereby forming an image on the recording medium.
A center axis (hereinafter, referred to as an optical axis of an incident system) of a laser beam outputs from the laser beam source has a minute angular misalignment with respect to a designed angle due to variations in dimensions of components or variations in assembly between components. When the optical axis of the incident system has an angular misalignment, an incident position and the incident angle with respect to the lens is uniformly displaced with respect to designed values. Consequently, an image-forming performance with respect to the photosensitive member is impaired. In particular, when a direction in which a laser beam deflected by the deflection unit scans on the photosensitive member on the optical scanning device is referred to as a primary scanning direction, and a direction orthogonal thereto is referred to as a secondary scanning direction, a phenomenon that the optical axis of the incident system inclines in the secondary scanning direction occurs. With such a phenomenon, the incident position of the laser beam with respect to the lens is changed. The optical scanning device is designed to allow a laser beam to pass through the lens at a position having a high light-converging performance. However, if the incident position of the laser beam into the lens varies, the laser beam cannot converge adequately on the photosensitive member, and thus deterioration of an image quality of an output image may occur.
In order to solve the above-described problem, Japanese Patent Laid-Open No. 2007-248686 discloses an optical scanning device provided with an adjusting mechanism configured to incline a holding member configured to hold the laser beam source with respect to a mounting member in the secondary scanning direction.
In a case where the angular misalignment occurs with respect to the designed angle due to variation in dimension of components or variations in assembly between the components, and thus changing of the angle of the optical axis of the laser beam in the secondary scanning direction is desired, a configuration of Japanese Patent Laid-Open No. 2007-248686 can realize a stable improvement of an optical performance by adjusting the holding member in a direction of cancelling the angular misalignment without increasing component accuracy and assembly accuracy. A principle of adjustment is as follows. The holding member and the mounting member are fixed with a screw in a state of having an elastic member therebetween. When assembling the optical scanning device of Japanese Patent Laid-Open No. 2007-248686, an operator corrects an optical path of the laser beam by rotating a screw and changing a posture of the holding member by a reaction force of the elastic member.
However, adjusting the optical path of the laser beam by a tightening amount of the screw by the intermediary of the elastic member between the holding member and the mounting member, which are desired to be fixed tightly under normal circumstances means, in other words, that tightening torque of the screw generated therebetween is reduced. Fixation of the screw may be loosened by environmental variations such as vibrations or temperature increase of a main body of the image forming apparatus, and hence the adjusted optical axis of the optical path may be deviated.
In view of such circumstances, direct fixation of the holding member to the mounting member with a screw without an intermediary of the elastic member is required in order to improve a performance of the optical scanning device.
However, in order to achieve a high accuracy of the optical axis with the fixation of the holding member with the screw, a next phenomenon needs to be overcome as a new challenge. In other words, a mounting surface, which is a portion where the holding member and the mounting member come into contact with each other, and which determines the posture of the holding member, is difficult to be machined into a completely flat surface even though an attempt is made to machine the surface with a highest possible degree of accuracy. With such a slight distortion of planarity and a difference in flatness between different mounting seat faces, the seat face of the holding member is forcedly adapted to the mounting member by a strong thrust force of the screw. At this time, stress is generated at each of a plurality of fixing points with the screws. In other words, when the holding member is mounted on the mounting member with the screw, the holding member is deformed slightly, and the deformation causes a deviation of the optical path of the laser beam.